Fuel cell technology shows great promise as an alternative energy source for numerous applications. Several types of fuel cells have been constructed, including polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For a comparison of several fuel cell technologies, see Los Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power by Sharon Thomas and Marcia Zalbowitz.
FIG. 1 represents an example of a fuel cell 100, including a high surface area anode 110 including an anode catalyst 112, a high surface area cathode 120 including a cathode catalyst 122, and an electrolyte 130 between the anode and the cathode. The electrolyte may be a liquid electrolyte; it may be a solid electrolyte, such as a polymer electrolyte membrane (PEM); or it may be a liquid electrolyte contained within a host material, such as the electrolyte in a phosphoric acid fuel cell (PAFC).
In operation of the fuel cell 100, fuel in the gas and/or liquid phase is brought over the anode 110 where it is oxidized at the anode catalyst 112 to produce protons and electrons in the case of hydrogen fuel, or protons, electrons, and carbon dioxide in the case of an organic fuel. The electrons flow through an external circuit 140 to the cathode 120 where air, oxygen, or an aqueous oxidant (e.g., peroxide) is being fed. Protons produced at the anode 110 travel through electrolyte 130 to cathode 120, where oxygen is reduced in the presence of protons and electrons at cathode catalyst 122, producing water in the liquid and/or vapor state, depending on the operating temperature and conditions of the fuel cell.
Hydrogen and methanol have emerged as important fuels for fuel cells, particularly in mobile power (low energy) and transportation applications. The electrochemical half reactions for a hydrogen fuel cell are listed below.
Anode:2H2→4H+ + 4e−Cathode:O2 + 4H+ + 4e−→2H2OCell Reaction:2H2 + O2→2H2OTo avoid storage and transportation of hydrogen gas, the hydrogen can be produced by reformation of conventional hydrocarbon fuels. In contrast, direct liquid fuel cells (DLFCs) utilize liquid fuel directly, and do not require a preliminary reformation step of the fuel. As an example, the electrochemical half reactions for a Direct Methanol Fuel Cell (DMFC) are listed below.
Anode:CH3OH + H2O→CO2 + 6H+ + 6e−Cathode:1.5O2 + 6H+ + 6e−→3H2OCell Reaction:CH3OH + 1.5O2→CO2 + 2H2O
The reaction schemes illustrate the production of water at the cathode during operation of these fuel cells. The water content of the cathode during fuel cell operation is affected by factors including a) the production of water due to the normal course of the reduction reaction at the cathode, b) production of water from the oxidation of fuel that has crossed through the electrolyte to the cathode instead of reacting at the anode, and c) water flux from the electrolyte to the cathode. The contribution of water flux to the water content at the cathode may be affected by factors including a) molecular diffusion due to a concentration gradient between the electrolyte and the water at the cathode, b) water permeation due to the hydraulic pressure difference between the electrolyte and the water at the cathode, and c) electro-osmotic drag associated with proton (H+) transport to the cathode.
If allowed to accumulate, liquid water at the cathode can severely limit the rate at which gaseous oxidant reaches the catalyst surface. This can result in an undesirable condition referred to as “cathode flooding” in which at least a portion of the cathode catalyst is blocked from contact with oxidant gas due to the presence of liquid on the catalyst. Consequently, water is typically removed from the cathode as vapor and/or liquid in the oxidant gas flow stream. This water either is vented from the system or is further condensed external to the fuel cell. If desired, the recovered water may then be supplied back to the anode for humidification of the anode stream and/or reaction with the fuel at the anode catalyst. Water lost from the fuel cell typically must be replaced in order to keep the cell in stoichiometric neutrality. If excess water in either the liquid or gaseous phase is vented from the system, then additional water typically is provided to the fuel cell to avoid dehydration. If a significant amount of water vapor must be condensed for recycling within the system, the fuel cell can have undesirable parasitic losses associated with high operating oxidant stoichiometries. Moreover, the presence of an external condenser that is large relative to the entire system can introduce additional weight, parasitic losses, and complexity to the fuel cell and the system incorporating the fuel cell.
The performance of conventional DMFCs may also suffer due to “methanol crossover,” in addition to cathode flooding. The material used to separate the liquid fuel feed from the gaseous oxidant feed in a DMFC typically is a stationary PEM that is not fully impermeable to methanol or other dissolved fuels. As a result, methanol fuel may cross over the membrane from the anode to the cathode, reacting with the cathode catalyst directly in the presence of oxygen to produce heat, water and carbon dioxide, but no useable electric current. In addition to being an inherent waste of fuel, methanol crossover causes depolarization losses due to a mixed potential at the cathode and, in general, leads to decreased cell performance.
It is desirable to provide a system for controlling and reducing the amount of water at or within the cathode in a fuel cell, particularly for a fuel cell that uses air or oxygen as the oxidant. Preferably such a system would prevent and/or buffer the system against cathode flooding and, if needed, would recover the water produced by the fuel cell in order to maintain water neutrality without the addition of significant parasitic losses or of increased system complexity. It is also desirable to provide a fuel cell in which fuel crossover is minimized.